1. Field of the Invention
The present invention relates to a process for preparing polymer latex resin powder, and particularly to a process for preparing polymer latex resin powder comprising the steps of coagulating polymer latex and aging it, wherein the process comprises a step of introducing an emulsifier into polymer latex emulsion before coagulation.
2. Description of the Related Art
In general, an emulsion polymerization process for continuously preparing polymer resin in latex into powder includes a coagulation process and an aging process. The coagulation process is occurred by introducing an aqueous solution of coagulant such as inorganic salt or acid, etc. into polymer latex. As a result, the electrostatic stabilization by emulsifier is broken and polymer particles in latex conglomerate by neutralization of electric charge, which is referred to as floc. However, since such floc cannot form a physical bond but form a kind of multi-particle assemblage, break-up may easily occur by agitating flow due to external impeller, etc. Therefore, in order to increase strength of floc, the aging process follows the coagulation process. The aging process is conducted at high temperature and floc forms one particle having some strength by mutual penetration between chains.
However, since such process uses an excessive amount of coagulant and thus coagulation process is very rapid to make it significantly irregular, final particles have irregular shape and significantly broad particle size distribution, which causes many problems in process. Practically, if large particles with a diameter of 400 μm or more are largely produced, problems in transfer and storage occur. And, if fine powders with a diameter of 70 μm or less are largely produced, dehydration and drying capacities drop, problems are raised in loss of resin due to scattering of fine powders and transfer of powder during process, and dust is produced to cause environmental pollution and aggravation of work conditions.
In order to improve these problems, U.S. Pat. No. 4,897,462 and British Patent No. 2,157,97A have described slow coagulation at critical coagulation concentration or less, but the method cannot overcome high viscose area generally produced in the slow coagulation and thus it is applied only for batch-coagulation process. And, moisture contents of powder are high due to slow coagulation and thus dehydration effects are inferior, and powder of which structure is not fine may be produced because of irregular mutual melting of fine solid particles due to high temperature during aging process, and consequently, bulk density of powder may be lowered.
In addition, Japanese Laid-open Publication Sho 53-33244, Sho 53-137873, Sho 56-41227, Sho 57-59929, Sho 58-87102, and Sho 61-42923, etc. have described a method for recovering powder by dispersing droplets of a polymer latex in a coagulating atmosphere by gas-phase spray system.
Although this method showed superior results in terms of particle size distribution, particle is attached to the wall surface of coagulation tank, and, due to a large amount of recovery solution for preventing such particle attachment, solid contents control inside a coagulation tank is limited and waste water is produced. And, for commercial application, coagulation tank equipment occupies very large area and space and thus energy consumption for satisfying coagulation condition of high temperature of 60° C. or more increases much, and coagulant should be introduced in large amount and thus remaining coagulant may have a bad influence on final quality of polymer resin.
In addition, U.S. Pat. Nos. 4,997,911 and 5,065,938 have described a method for controlling size of powder in the presence of an organic solvent and a method for improving bulk density, but used amount of an organic solvent is very large, which is a burden on realization of process equipment.